1. Field of the Invention
This invention relates to a system for controlling fuel metering in an internal combustion engine, more particularly to a system for controlling fuel metering in an internal combustion engine wherein the amount of fuel injection is optimally determined over entire range of engine operating conditions including engine transients using an intake air model and by simplifying its calculation, while coping with various instances such as system degradation and initial manufacturing variances.
2. Description of the Prior Art
In a conventional fuel metering control system, the fuel injection amount was usually determined by retrieving mapped data predetermined through experimentation and stored in advance in a microcomputer memory using parameters having high degrees of correlation with the engine cylinder air flow. As a result, the conventional technique was utterly powerless to cope with the parameters' change which had not been taken into account at the time of preparing the mapped data. The same difficulty could also be encountered due to the degradation and initial manufacturing variance etc. in the fuel metering control system. Further, since the mapped data were intrinsically prepared solely focussing on steady-state engine operating conditions and transient conditions were not described there, the conventional technique was unable to determine the fuel injection amount under engine transients with accuracy. For that reason, there are recently proposed techniques to establish a fluid dynamic model describing the behavior of the air intake system so as to accurately estimate cylinder air flow such as disclosed in Japanese Laid-Open Patent Publication 2(1988)-157,451 or U.S. Pat. No. 4,446,523.
Also the assignee proposed in Japanese Patent Application 4(1992)-200,330 a method for estimating cylinder air flow by determining the air mass flow past the throttle while treating the throttle as an orifice to establish a fluid dynamic model based on standard orifice equations for compressible fluid flow. The fluid dynamic model used there is, however, premised on an ideal state and requires various assumptions. It is therefore impossible to wipe out all the errors which could be introduced at the time of modeling. Further, since it is quite difficult to accurately determine constants such as specific-heat ratio used in the model, errors arising possibly therefrom may disadvantageously be accumulated. Furthermore, the equations necessitate calculation of powers, roots or the like. Since approximate values are used for them in practice, resulting additional errors.